Near-field Raman: Imaging Beyond the Diffraction Limit
Introduction
Specialized applications can require Raman imaging capability with a resolution
below the diffraction limit of classical optics (< 200 nm). Near-field Raman imaging
is a correlative microscopy technique thatlinks chemical Raman information withdata acquired through high-resolution
Scanning Near-field Optical Microscopy(SNOM). Typically, lateral resolutions of lessthan 100 nm can be achieved. The combination of a high-throughput spectroscopic
system and the cantilever-based SNOM
technique provides unrivaled sensitivity
and imaging quality in one microscope
setup. Microscope systems that combine
several measurement techniques offer
a more comprehensive characterization
and allow the strengths of the individual
approaches to complement each other.
Near-field Raman Imaging Principle
The excitation laser light is focusedthrough the SNOM-tip resulting in a“near-field” (evanescent field) on the far
side of the aperture. While the sample
is moved with a piezo-driven scan stage,
the transmitted light is spectroscopicallyacquired point by point and line by line inorder to generate a hyperspectral image.
The optical resolution is therefore limited
only by the diameter of the aperture(< 100 nm). Using a beam deflection setup,
as in AFM contact mode, ensures thatthe cantilever remains in contact withthe sample. Therefore the topography
can be recorded simultaneously with thenear-field Raman measurement.

Working principle of WITec near-field Raman
imagingParameters of the near-field Raman measurement

Experimental Results
Raman imaging has been proven usefulto study spatially resolved chemical andphysical features of graphene. It has beenshown that strain variations at the nanoscale effect graphene‘s quality (1, 2, 3)
Here, an exfoliated graphite flake sample
was investigated with near-field Raman
microscopy. Fig. 1 shows the sample
illuminated by white light. At one point areference Raman spectrum was acquired
through the SNOM tip with 5 sec. integration time per spectrum (Fig. 2). The Raman
signal of the graphite flake was analyzed
in terms of the integrated intensity of theG-Band (near 1600 rel./cm) along a line
depicted in red in Fig. 1. The width of thegraphite flake can be determined by plotting the increase in the intensity relative tothe distance. The complete length of theThe WITec alpha 300 RS Microscopeline trace was 20 µm with 400 measurement points and 1 sec. integration time
per spectrum (Fig. 3). Single measurement
points can be distinguished through aline-trace zoom-in on the edge of the graphite flake. A lateral resolution of < 100 nmof the near-field Raman technique canthus be established (Fig. 4). Fig. 5 shows
the topography image of the exfoliatedgraphite recorded at the same time asthe near-field Raman measurement. Thecorresponding topography curve along
the blue line shows a height of ~10 nm
and the small size of the graphite flake(Fig. 6). With the same scan a near-field
Raman image of the G-Band intensity canbe acquired (Fig. 7). Due to the extreme
sensitivity of the measurement technique,a laser power of approx. 5 µW at the sample and an illuminated area of less than
100 nm was sufficient for image generation. At each image pixel a complete Raman
spectrum was acquired with only 0.53 sec.
integration time per spectrum. The scan
range was 5 µm x 1.7 µm and the image
size 100 x 35 pixels. The G-band intensity
along the red line reveals the measurable
signal variations between the small sample and the substrate (Fig. 6).

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Summary and Conclusion
WITec’s near-field Raman imaging systems
enable the generation of Raman images
with an optical resolution below thediffraction limit. The distribution of thechemical and molecular components canbe imaged at very high resolution. The throughput and optimized detection setup
allow very small sample volumes to beanalyzed at excitation energies that avoid
sample damage. Topography is also acquired simultaneously with the hyperspectral
image. Near-field Raman imaging pairs
unrivaled sensitivity with the ease-of-use
and reliability of the established cantilever
SNOM probe technique within a single
microscope system and is suitable for all
fields of application in which a detailed
sample characterization is required.

Literature

1) K. S. Novoselov et al. , Nature 2012, 490: 192.
2) A. Ferrari et al., Phys. Rev. Lett. 2006, 97: 187401
3) C. Neumann et al., Nature Comm. 2015, 6: 8429.

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Near-field Raman measurements of exfoliatedgraphite
1. Video image of an exfoliated graphite flake
2. Raman spectrum acquired though theSNOM-tip
3. Line scan showing integrated intensity ofthe G-Band
4. Line trace zoom-in
5. Topography image of exfoliated graphite(scale bar = 1 µm)
6. Topography curve
7. Near-field Raman image (scale bar = 1 µm)
8. G-band intensity.